1. Field of the Invention
The present invention relates to an optical fiber for which the effective core cross-sectional area is enlarged in order to transmit high-power light and the like, and particularly, to a microstructure fiber including a wholly solid structure having fine high refractive index scatterers disposed in a dispersed manner in cladding areas that surround a core area (solid photonic band gap fiber), and an optical fiber module having the optical fiber.
Furthermore, the invention relates to a technique that maintains propagation of light in the optical fiber as single mode propagation, and enlarges the effective core cross-section of the optical fiber.
In addition, the invention relates to a fiber amplifier or a fiber laser which introduces excitation light into an optical fiber, amplifies signal light using induced emission caused by the excitation light, or oscillates and outputs a laser in technical fields of optical amplification and optical oscillation, an optical fiber in which the fiber amplifier or a fiber laser is preferably used, and an optical fiber module.
2. Description of the Related Art
When optical fibers are classified according to transmission modes, optical fibers are classified into multi-mode fibers and single mode fibers.
In terms of transmission characteristics, single mode fibers having characteristics such as a small transmission loss are overwhelmingly advantageous, and particularly in optical fibers used in fiber amplifiers or fiber lasers, an effect of improving the beam qualities of an output beam can be obtained by inhibiting high-order mode propagation, that is, substantially using a single mode fiber which realizes single mode propagation.
In addition, in recent years, remarkable advances have been made regarding techniques that increase the output of fiber amplifiers or fiber lasers.
Accompanying the development of techniques to increase the output of fiber amplifiers or fiber lasers, there has been a demand for optical fiber-type components including rare earth element-added fibers that are used for fiber amplifiers and fiber lasers to have resistance with respect to high-power light.
With regard to optical power, characteristics of an optical fiber to which attention should be paid are generally known to be optical damage and non-linear optical effects.
Both optical damage and non-linear optical effects are phenomena occurring when the power density (the optical power per unit light guide cross-sectional area) of light is high.
Therefore, the power density of light needs to be decreased in order to obtain high-output light while avoiding appearance of these undesirable phenomena.
In addition, the cross-sectional area through which light passes needs to be large in order to decrease the power density without decreasing the output power.
Here, as a general index of the light guide cross-sectional area, the definition of a so-called effective core cross-sectional area is used.
The effective core cross-sectional area Aeff is defined using the following formula (1).
                              [                      Formula            ⁢                                                  ⁢            1                    ]                ⁢                                                                                                A          eff                =                              2            ⁢                                          π                ⁡                                  [                                                            ∫                      0                      ∞                                        ⁢                                                                                                                                                  E                            ⁡                                                          (                              r                              )                                                                                                                                2                                            ⁢                      r                      ⁢                                              ⅆ                        r                                                                              ]                                            2                                                          ∫              0              ∞                        ⁢                                                                                                  E                    ⁡                                          (                      r                      )                                                                                        4                            ⁢              r              ⁢                              ⅆ                r                                                                        (        1        )            
Where, in formula (1), E(r) indicates the electric field distribution of light inside an optical fiber, and r indicates the distance of the optical fiber in the radius direction.
Therefore, in recent years, a variety of active attempts for enlarging the effective core cross-sectional area have been made as described in, for example, Non-Patent Documents mentioned below.
Proc. of SPIE vol. 5335, p. 132-139 (2004) discloses a method of enlarging the effective core cross-sectional area by changing the shape of the refractive index distribution in the core of an optical fiber.
However, in that method, since the cutoff wavelength increases as the effective core cross-sectional area enlarges, there is a problem in that a tradeoff exists between single mode propagation necessary to maintain the beam qualities and enlargement of the effective core cross-sectional area.
In addition, in the refractive index distribution disclosed in Proc. of SPIE vol. 5335, p. 132-139 (2004), in a case that a fiber is used in a bent state, there is a problem in that the effective core cross-sectional area significantly decreases, (regarding the behaviors of the effective core cross-sectional area in a case that a fiber is bent, Opt. Express, 14, p. 69-81 (2006) discloses detailed investigation results).
In addition, Opt. Lett., vol. 25, p. 442-444 (2000) discloses a method which can substantially realize single mode propagation in a multi-mode fiber having a large effective core cross-sectional area by using a fiber having a high-order mode, and using the fiber in a bent state so as to cause bending loss in the high-order mode.
This method is being relatively widely used; however, as described in Proc. of OFC/NFOEC 2008, OTuJ2 (2008), the method also has a certain limitation in enlarging the effective core cross-sectional area since the effective core cross-sectional area decreases when a fiber is bent.
Therefore, there is a problem in that the effective core cross-sectional area cannot be sufficiently enlarged.
Opt. Express, 14, p. 2715-2720 (2006) and Proc. of ECOC 2008, Th. 3.C. 1 (2008) disclose a method in which the effective core cross-sectional area is enlarged using a photonic crystal fiber and a method in which the effective core cross-sectional area is enlarged by decreasing the relative refractive index difference respectively.
These methods can realize a larger effective core cross-sectional area than in the related art, but fibers used in all methods are difficult to bending, and therefore it is not possible to use the fibers in a bent state.
Therefore, it is not possible to realize a compact fiber amplifier or fiber laser.
Additionally, Proc. of CLEO/QELS 2008, CPDB6 (2008) discloses a method in which the effective core cross-sectional area is enlarged using a leakage fiber; however, similarly to the methods in Opt. Express, 14, p. 2715-2720 (2006) and Proc. of ECOC 2008, Th. 3.C. 1 (2008), a leakage fiber is difficult to bending, and there is another problem in that it is difficult to increase the oscillation efficiency of a laser or the amplification efficiency of an amplifier since the transmission loss is, in principle, large.
Proc. of OFC/NFOEC 2008, OWU2 (2008) and Opt. Express, 13, p. 3477-3490 (2005) disclose methods in which a high-order mode is removed by combining only the high-order mode to the periphery of the core of a fiber so as to substantially realize single mode propagation.
These methods can effectively remove the high-order mode, but the refractive index distribution and the structure are extremely complicated such that there is a demand for an extremely sophisticated control.
Therefore, there are problems in that manufacturing is difficult, the costs are high, the yield is low, and the like.
By the way, in recent years, a photonic band gap fiber which is based on a different optical propagation mechanism from those of optical fibers of the related art has been attracting attention as an optical fiber suitable for high-output fiber lasers or fiber amplifiers.
The photonic band gap fiber has a structure in which, basically, the Bragg reflection of light is used, and a plurality of fine high refractive index scatterers are disposed in cladding areas in the periphery of the core area formed of a material having a low refractive index so as to have a periodic structure.
In addition, it is possible to use a photonic band gap (PBG) with respect to out-of-plane propagation light formed by the periodic structure of the high refractive index portions in the cladding areas by having the above structure, and it is possible to confine optical waves to the core area (low refractive index portion) which is a defect portion with respect to the periodic structure and to propagate light in the longitudinal direction of the fiber.
Furthermore, a solid photonic band gap fiber with which the photonic band gap fiber can be manufactured in a solid structure is being developed (for example, refer to Proc. of SPIE vol. 5335, p. 132-139 (2004)).
The solid photonic band gap fiber has a structure in which, on a cross-section perpendicular to the longitudinal direction, basically, the core area is disposed in the central portion, the cladding areas are disposed so as to surround the core area, and the high refractive index portions are disposed in the cladding areas so as to surround the core area and have a lamellar periodic structure.
In addition, in the solid photonic band gap fiber, the core area is formed of a solid substance having a relatively low refractive index (generally, silica glass is used), the base portions of the cladding areas are formed of the same solid substance having a relatively low refractive index as for the core area (generally, silica glass is used), and the high refractive index portions are formed of a plurality of fine high refractive index scatterers (generally, a material obtained by doping a refractive index-increasing substance in silica glass is used).
Even for the solid photonic band gap fiber, the effective core cross-sectional area is studied in, for example, Opt. Express, 16, p. 11735-11740 (2008) and the like.
In Opt. Express, 16, p. 11735-11740 (2008), a result is reported that single mode propagation can be realized in a case that the mode field diameter (MFD), which is the same index as the effective core cross-sectional area, is 19 μm to 20 μm in the solid photonic band gap fiber.
However, it is reported that, in when an attempt is made to manufacture an effective core cross-sectional area in which MFD is 19 μm to 20 μm, it is difficult to realize single mode propagation through high-order mode propagation (refer to The Institute of Electronics, Information and Communication Engineers, Proceedings of the IEICE Society conference, BS-7-8 (2009)).
Furthermore, it is reported that, in the structure of the photonic band gap fiber disclosed in The Institute of Electronics, Information and Communication Engineers, Proceedings of the IEICE Society conference, BS-7-8 (2009), the bending loss of the fundamental mode is large, and it is difficult to bend the fiber in a compact size and to use the fiber.
Also, none of Proc. of SPIE vol. 5335, p. 132-139 (2004), Opt. Express, 14, p. 69-81 (2006), Opt. Lett., vol. 25, p. 442-444 (2000), Proc. of OFC/NFOEC 2008, OTuJ2 (2008), Opt. Express, 14, p. 2715-2720 (2006), Proc. of ECOC 2008, Th. 3.C. 1 (2008), Proc. of CLEO/QELS 2008, CPDB6 (2008), Proc. of OFC/NFOEC 2008, OWU2 (2008), and Opt. Express, 13, p. 3477-3490 (2005) disclose photonic band gap fibers, particularly, solid photonic band gap fibers which are the subject of the invention, but disclose optical fibers having different propagation methods.
Basically, when the propagation methods of light are different as described above, even when the methods described in above Non-Patent Documents are effective for optical fibers, it is not always true that the methods are effective in a case that the methods are applied to a photonic band gap fiber, particularly the solid photonic band gap fibers which are the subject of the invention.
As described above, in techniques of the related art, enlargement of the effective core cross-sectional area and realization of single mode propagation through removal of a high-order mode are conflicting objects, that is, objects on a tradeoff relationship, and these objects have not yet been achieved even in, particularly, the solid photonic band gap fiber.
Since the invention has been made in consideration of the above circumstances, an object of the invention is to provide a solid photonic band gap fiber, that is, an optical fiber in which high-order mode propagation is effectively inhibited so as to substantially maintain single mode propagation and enlarge the effective core cross-sectional area in a microstructure fiber in which fine high refractive index scatterers are disposed in a dispersed manner in cladding portions, and light is transmitted using a photonic band gap, a fiber module using the solid photonic band gap fiber, furthermore, a fiber amplifier, and a fiber laser.